Concerns about the availability of fossil fuel energy resources and pollution caused by the use of gas, oil, coal, and nuclear power have prompted research to attempt to develop viable alternate energy sources such as solar, wind, and geothermal energy. For illustration purposes, the following discussion focuses on the techniques and problems associated with solar energy. However, it is understood that the present invention can be utilized with wind and geothermal energy, as well as with conventional sources of power.
Since solar energy is pollution free, has an inexhaustible source, and can be captured with comparatively inexpensive equipment, it is a very promising alternate energy source. Solar energy is broadly defined as electricity which is generated from radiant light (electromagnetic energy) of the sun striking a photovoltaic cell.
Although solar energy is a promising energy source, one drawback of solar energy is that by its very nature it is a variable power source. This variability is caused not only by the lack of a power source during the night, but also by changes in the incidence of light relative to the earth's surface during seasonal change, and by atmospheric variables such as changing weather conditions, smog, dust, and the like. This variable nature of solar energy creates difficulties, particularly since most energy and power usages require dependable and continuous sources of energy. Thus, solar energy must be stored to make it available when required.
Various techniques of storage have been developed. One common storage technique is the use of storage batteries. However, storage batteries are expensive, difficult to maintain, of limited life, and the toxic substances used in batteries create a disposal problem when the battery storage capacity is exhausted.
Another storage technique uses the electrical energy generated from a photovoltaic cell to produce hydrogen gas from water, and the hydrogen is stored and later used to produce energy when needed. The production of hydrogen from water generally consists of transmitting electrical energy to electrodes within an electrolyzer to induce an electric potential difference which disassociates water into hydrogen and oxygen. The electrolyzer generally contains pure water having as electrolyte of sodium hydroxide or potassium hydroxide. These electrolytes are not destroyed nor do they need to be replenished during the operation of the electrolyzer. Thus, even though the electrolysis action may take place intermittently, the hydrogen produced can be maintained in storage and turned back into electrical energy (either by combustion or by use of a fuel cell) during times of little or no solar radiant light. Therefore, the variable character of solar energy will have no or little affect on the desired electrical output.
One of the more efficient electrolyzers presently available is a solid polymer electrolyte ("SPE") unit. These units basically consist of two electrodes, an anode and a cathode, placed in a perfluorinated sulfonic acid polymer. The electrodes are connected through an external circuit to a power supply. Water is broken down at the anode into oxygen, hydrogen ions and electrons. The electrons flow through the external circuit to the cathode while the hydrogen ions flow through the electrolytic polymer to the cathode where they combine with the electrons and form hydrogen. The equations at the anode and cathode are: EQU H.sub.2 O.fwdarw.2H.sup.+ +1/2O.sub.2 +2e.sup.- EQU 2H.sup.+ +2e.sup.- .fwdarw.H.sub.2
The overall reaction is thus: EQU H.sub.2 O.fwdarw.H.sub.2 +1/2O.sub.2
The by-product of this process is an effluent containing trace hydrofluoric acid, oxygen gas and excess water.
SPE electrolyzers are one of the two main types of electrolyzers available. SPE electrolyzers are also known as PEM, or Proton Exchange Membrane, for the way in which they split water. The other type, liquid electrolyte ("LE") electrolyzers, uses as its electrolyte a strong acidic or basic solution, typically potassium hydroxide. However, there are a number of advantages that an SPE electrolyzer has over LE electrolyzers. The concentration of the solution in an LE electrolyzer must be maintained at a constant level for the electrolytic reaction to take place, while SPE electrolyzers maintain constant concentration over their life. SPE electrolyzers are also safer, since they do not require a supply of a strong highly corrosive basic solution as do LE electrolyzers.
The hydrogen gas produced is a storable, transportable, clean, and non-polluting fuel. However, hydrogen has the fundamental limitation of being difficult to store. Hydrogen has a boiling point of -252.87.degree. C. and a density of 0.09 grams per liter. This means that in order to store hydrogen in reasonable sized tanks, it must be stored either under pressure, at low temperature, or both. Unfortunately, it takes energy to create high pressures and low temperatures. Thus, the overall efficiency and cost effectiveness of producing and storing hydrogen is reduced.
In order to overcome the hydrogen storage problem, it has been found that hydrogen can be stored in a solid form via "rechargeable" metal hydrides, such as iron-titanium-manganese (Fe.sub.44 Ti.sub.55 Mn.sub.5) alloy, mischmetal-nickel aluminum hydriding (Mn.sub.0.97 Ni.sub.4.5 Al.sub.0.5) alloy, and the like. This can best be described by the reversible chemical reaction of a solid metal hydride(Me) with gaseous hydrogen (H.sub.2) to form a solid metal hydride (MeH.sub.x): ##EQU1##
The forward or exothermic reaction is characteristic of the charging (absorption) of hydrogen to the hydride while the reverse or endothermic reaction is the discharging (desorption) of hydrogen from the metal hydride. Among the many advantages of hydrogen storage via a metal hydriding alloy, the most significant is the low charging and discharging pressures required to hydride which lessens the risk of leakage and explosion associated with storing hydrogen as a compressed gas.
When examining the thermodynamic aspects of the reversible metal-hydrogen reaction, it is advantageous to determine the absorption and desorption properties of metals from pressure-composition isotherms. The abscissa of such isotherms is typically in the form of a hydrogen atoms to metal atoms ratio ("H/M"). FIG. 1 shows the ideal absorption-desorption pressure-composition isotherm for a metal-hydrogen system where the plateau pressure ("P.sub.p ") 30 is shown connecting points B and C. Once the plateau pressure is reached, the majority of the absorption or desorption of hydrogen takes place at this constant pressure P.sub.p. The curves connecting points A and B as well as points C and D show that for a large increase or decrease in pressure, the amount of hydrogen absorbed or desorbed is small.
In reality, while such isotherms as shown in FIG. 1 might be achievable, most hydrides deviate from this ideal behavior. In addition to the fact that the plateau region slopes and the boundaries of this region are not as well defined, there also exists hysteresis between absorption and desorption curves. For ideal hydrides, there is no means by which to measure the composition of the hydride when located along the plateau pressure; but the slope in the isotherm for real metals makes finding the hydrogen to metal ratio as simple as knowing the temperature and pressure of the hydride.
The plateau pressure P.sub.p is related to the absolute temperature of the reaction, T.sub.R, by the Van't Hoff equation: ##EQU2## where .DELTA.H is the change in enthalpy, .DELTA.S is the change in entropy and R.sub.u is the universal gas constant. From the Van't Hoff relationship one can determine the charging and discharging pressures and temperatures of the tank.
Experimental work into hydrogen storage for photovoltaic systems has been performed at Brookhaven National Laboratories. This experimental work has been published in Schoener et al., "An Integrated Test Bed For Advanced Hydrogen Technology: Photovoltaic Array/Electrolyzer System," BNL51577 Brookhaven National Laboratory (1982), Leigh et al., "Photovoltaic-electrolyzer System Transient Simulation Results," BNL40081 Brookhaven National Laboratory (1983), and Metz et al., "Photovoltaic-powered Solid Polymer Electrolyte (SPE) Electrolyzer System Evaluation," BNL51940 Brookhaven National Laboratory (1982). The system of the prior art consisted of a 5 kW photovoltaic array, a 15 kW advanced technology electrolyzer, and an iron-titanium hydride tank with a storage capacity of 50 lbs or 8600 standard ft.sup.3 of hydrogen. This Brookhaven system used a power conditioner between the photovoltaic array and electrolyzer that also drew power from local utilities to supplement the 5 kW array. This was done to allow the advanced technology electrolyzer to be operated off the utilities for baseline constant power testing, and to simulate a 100% solar power source. It was found that use of an active power conditioner would entail losses ranging from 21-29%.
A similar analytical model for a photovoltaic array-electrolyzer system was constructed at the Institute for Technical Physics in West Germany. The results from this model were published in Carpetis, C., "An Assessment of Electrolytic Hydrogen Production by Means of Photovoltaic Energy Conversion," Int'l J. Hydrogen Energy, Vol. 9, No. 12 (1990) pp. 969-991. From the Carpetis model, it was determined that the use of a power conditioner would not improve system performance, since it would increase power losses from 5% without conditioning to 5-10%.
Since 1990, the Department of Technical Physics at the Helsinki University of Technology has undertaken a three-year project to investigate hydrogen energy technologies for solar energy systems at high latitudes. The results of this project were published in Kauranen et al., "Hydrogen Energy Storage for Photovoltaic Systems," Helsinki University of Technology, Dept. of Technical Physics (1990). The goal of the project was to develop the required technologies and demonstrate the technical feasibility of a hydrogen storage system with electrolytic hydrogen production and electrical power production using hydrogen fuel. A pilot system is currently being constructed for a 1-4 kW-hr daily load. The system disclosed uses battery storage and pressurized hydrogen storage, and achieves load matching by switching components on and off.
A similar project was also undertaken at Humboldt State University in Arcata, Calif., with the Schatz Solar Hydrogen Project. The results of the California project were published in "Project Hydrogen '91: Technical Proceedings," Amer. Acad of Science (1992). The California system consisted of a 9.2 kW photovoltaic array connected in parallel to a 24 VDC, 37-A.hr nickel-cadmium battery and a Teledyne Energy ALTUS 20.TM. liquid potassium-hydroxide electrolyte electrolyzer capable of producing 20 slpm of hydrogen and 10 slpm of oxygen. The hydrogen and oxygen were used to drive an Ergenics 1.0 kW fuel cell, and when the fuel cell was not in use, the hydrogen and oxygen were stored as compressed gases in separate tanks.
Finally, photovoltaic energy storage in the form of hydrogen fuel has also found application as a means of vehicle propulsion. One noteworthy project currently under development is the LaserCel.TM. prototype vehicle at The American Academy of Science. The results of this LaserCel.TM. project were published in "Project Hydrogen '91: Technical Proceedings," Amer. Acad of Science (1992). A subcompact car (modification of a Ford "Fiesta") was powered by a LaserCel.TM. inhouse-developed SPE hydrogen-air fuel cell, driving an electric motor coupled to the wheels using a standard transmission. The hydrogen was stored as a metallic hydride in tanks containing an iron-titanium-manganese (Fe.sub.44 Ti.sub.55 Mn.sub.5) alloy, and a gas compressor was used to raise the hydrogen and air to the operating pressure of the fuel cell. Heat exchange to extract or store hydrogen from the tanks was achieved using thermoelectric heat pumps, and the fuel cell could be operated in reverse as an electrolyzer to produce hydrogen (from a specially designed water supply built into the vehicle) for storage. Solar panels could be mounted on the vehicle so that it can literally produce its own fuel in this fashion (under favorable sunlight conditions). The vehicle was said to have a range of 300 km, extendable to 500 km, and great emphasis was made of the fact that hydrogen consumption in a fuel cell is two to three times more efficient than burning it in an internal combustion engine.
As it can be seen, although the technologies for the collection and use of photovoltaic energy, electrolysis, and the storage of hydrogen in various forms are established either theoretically or by actual applications, virtually no work has been done in combining the three technologies into an integrated hydrogen production and storage system for photovoltaic energy conversion to achieve a viable means for the production of hydrogen in an efficient manner.
Thus, there is a need for an apparatus for hydrogen production from alternate energy sources in an efficient manner and which is able to store hydrogen in a manner which does not require a substantial consumption of energy.